In all relationships and marriages it takes two to tango, and the person you have married to may and does, change... and often, not for the better.
There is a great deal of difference between things not working out in marriage(s) (Gingrich, Reagan) and craven skirt-chasing while being “faithfully” married (Clinton)...
Just as an aside, (not implying a cause and effect) but historically, in the last fifty years the only President who can be considered great / near-great / outstanding was also the only one who has been married more than once - Ronald Reagan.
One divorce can occur, especially in a very young person who marries.
But what’s the excuse for cheating on his second wife?